Recently, YAG (e.g., Y3Al5O12) crystals with rare-earth ions doped, like Yb:YAG, have become popular as a high-power laser medium, especially, as a high-power short-pulse laser medium. In particular, it is because Yb-based laser media have quantum defects less (e.g., 8.7% in the case of Yb:YAG) and have a wider band width for oscillation. These characteristics make it possible to use high-performance semiconductor lasers for excited-light source.
Meanwhile, since laser media with a certain type of rare-earth ions, such as Yb, doped have an energy level of the quasi 4 level structure, it is necessary to overcome the reabsorption at lower laser level and the small stimulated-emission cross section in order to cause them to oscillate. In addition, it is also important to cause the excited-light distribution to match to the space-distribution pattern (i.e., traverse mode) of oscillatory-light distribution in order to achieve satisfactory laser efficiencies. It is because laser lights are absorbed intensely by areas that are excited weakly in the laser (or gain) medium within a laser oscillator. In addition, the laser performance is sensitive to the temperatures of crystal.
For high-power Yb-based lasers, such as Yb:YAG, whose efficiency is good, active-mirror laser structures including thin disks or microchips are one of the appropriate designs. It is because these structures can not only decrease the thermal lens effect but also suppress the temperature rise in gain medium. In addition, since the light-path length of laser beam within the gain medium is short in the case of the active-mirror laser structures, the reabsorption loss can be minimized. However, there is such a problem that it is difficult for such a thin gain medium to achieve efficient excited-light absorptions.
A. Giesen, et al., have so far attained sufficient excited-light absorptions by means of reflecting excited lights 16 times repeatedly between laser crystals and plurality of mirrors being fixated in front of the laser crystals with use of a colinear excitation constitution that is similar to the end-face excitation (see Non-patent Literature No. 1, for instance).
However, the laser apparatus of A. Giesen, et al., had such a problem that the constitution was very complicated. Hence, Luis E. Zapata, et al., developed in order to solve that problem a solid laser apparatus in which Yb3+:YAG with 200-μm thickness was formed onto a quadrangular-prism-shaped clad with 1.3-mm thickness, thereby causing excited lights, which came from the resulting stacked-type semiconductor laser, to undergo wave guiding from the longitudinal direction of the clad (see Non-patent Literature No. 2, for instance).
In the laser apparatus of Luis E. Zapata, et al., however, the excited lights coming from the semiconductor laser propagate mainly in the thick undoped YAG plate, and accordingly only some of the excited lights are absorbed in the thin Yb:YAG gain medium. Consequently, it is impossible for the gain medium to absorb the intense excited lights that come from the semiconductor laser.
Meanwhile, side-face excitation core-doped microchip lasers solve those problems. An excited light, which has been introduced into the microchip from one of the side faces, propagates through a transparent planar waveguide by means of total reflection without any loss, and then converges onto the central Yb:YAG core area. Since the diameter of the core is several millimeters, most of the excitation energy, which passes through the core diametrically, is absorbed effectively even by one-way excitation. Moreover, since the excitation constitution is so simple that it is not necessary to insert the optical system for excitation in front of the microchip, the degree of freedom is high in designing the laser oscillator.
M. Tsunekane, et al., developed a side-face excitation core-doped microchip laser apparatus (see Non-patent Literature No. 3, for instance). This laser apparatus is one in which a disk-shaped gain medium, which is mounted on the central portion of a rectangle-shaped planar waveguide, is excited by excited lights, which come from a stacked-type semiconductor laser, through the four side faces of the rectangle-shaped planar waveguide.